Device for sensing sheet transport condition

Abstract

In an electrophotographic copier, while a document is transported toward a predetermined position on a glass platen, its lateral dislocation and skew with respect to a document transport path as well as an amount of the dislocation and a direction of the skew are commonly sensed by a single sensor. The single sensor is positioned adjacent to one of laterally opposite ends of the transport path and has sensing surface of a predetermined area. While the document moves past the sensing surface of the sensor, the sensor generates a signal which corresponds to an area of the sensing surface which is occupied by the sheet.

What is claimed is: 
     
       1. A device for sensing a transport condition of a sheet, which is being transported along a predetermined transport path, with respect to a transport direction, said device comprising: a single sensor located in the vicinity of one of laterally opposite ends of the transport path and having a sensing surface which extends over a predetermined area, said single sensor continuously generating a detection signal coresponding to an area of said sensing surface which is occupied by the sheet while the sheet moves past the sensing surface;   a plurality of comparators each being supplied with the detection signal to compare a level of the detection signal with different reference levels which are respectively assigned to said comparators, each of said comparators continuously generating a comparison signal only for a duration in which the level of the detection signal is higher than the reference level assigned thereto;   computer means for comparing a number of the comparators which are generating the comparison signals with a predetermined reference number of the comparators which generate comparison signals while the sheet is in proper transportation, and computing lateral dislocation of the sheet with respect to the transport direction and an amount of the lateral dislocation from a result of the comparison; and   a differentiator having an input connected to the single sensor and an output connected to the computer means for generating a variation rate signal which corresponds to a variation rate in the level of the detection signal output from the single sensor, the computer means being constructed to compare a first predetermined period of time for which an area of the sensing surface of the sensor occupied by the sheet while the sheet properly moves past the sensing surface remains maximum with a duration of one of the comparison signals output from the comparators which is output for a shortest duration, and deciding that the sheet is skewed with respect to the transport direction when the first predetermined period is shorter than the duration of said one comparison output.   
     
     
       2. A device as claimed in claim 1, in which the computer means is further constructed to, when skew of the sheet is detected, compare a second predetermined period, which is one half a period which the sheet takes to properly move past the sensing surface of the sensor, with a duration of the variation rate signal output from the differentiator, and to determine a direction of the skew of the sheet with respect to the transport direction from a result of the comparison. 
     
     
       3. A device as claimed in claim 1, in which the single sensor is located in the vicinity of the rightmost end of the transport path with respect to the transport direction, the computer means being further constructed to decide that the sheet is skewed rightwardly upwardly (leftwardly downwardly) with respect to the transport direction when said second predetermined period is longer than the duration of the variation rate signal and that the sheet is skewed leftwardly upwardly (rightwardly downwardly) when the second predetermined period is shorter than the duration of the variation rate signal. 
     
     
       4. A device as claimed in claim 1, in which the single sensor is located in the vicinity of the leftmost end of the transport path with respect to the transport direction, the computer means being further constructed to decide that the sheet is skewed leftwardly upwardly (rightwardly downwardly) with respect to the transport direction when said second predetermined period is longer than the duration of the variation rate signal and that the sheet is skewed rightwardly upwardly (leftwardly downwardly) when the second predetermined period is shorter than the duration of the variation rate signal. 
     
     
       5. A device for sensing a transport condition of a sheet, which is being transported along a predetermined transport path, with respect to a transport direction, said device comprising; a single sensor located in the vicinity of one of laterally opposite ends of the transport path and having a sensing surface which extends over a predetermined area, said single sensor continuously generating a detection signal corresponding to an area of said sensing surface which is occupied by the sheet while the sheet moves past the sensing surface;   a plurality of comparators each being supplied with the detection signal to compare a level of the detection signal with different reference levels which are respectively assigned to said comparators, each of said comparators continuously generating a comparison signal only for a duration in which the level of the detection signal is higher than the reference level assigned thereto;   computer means for comparing a number of the comparators which are generating the comparison signals with a predetermined reference number of the comparators which generate comparison signals while the sheet is in proper transportation, and computing lateral dislocation of the sheet with respect to the transport direction and an amount of he lateral dislocation from a result of the comparison; and   a differentiator having an input connected to the single sensor for developing a variation rate signal indicative of a variation rate in the level of the detection signal output from the single sensor, a flip-flop having an input connected to an output of the differentiator for developing an output when variation rate signal is positive, a clock pulse generator for generating clock pulses, AND gates common in number to the comparators and supplied with the clock pulses and comparison signals output from the comparators, and an AND gate for receiving an output of said flip-flop and the clock pulses, outputs of said AND gates being supplied to the computer means, the computer means being constructed to compare a first predetermined number of clock pulses which are output from the clock pulse generator for a period in which an area of the sensing surface of the sensor occupied by the sheet when the sheet properly moves past the sensing surface remains maximum with a number of clock pulses which are output from the clock pulse generator for a duration of one of the comparison signals output from the comparators which is output for a shortest duration, and deciding that the sheet is skewed with respect to the transport direction when the first predetermined number of clock pulses is smaller than the number of clock pulses output from the clock pulse generator for said duration.   
     
     
       6. A device as claimed in claim 5, in which the computer means is further constructed to, when skew of the sheet is detected, compare a second predetermined number of clock pulses output from the clock pulse generator for a period which is one half a period which the sheet takes to properly move past the sensing surface of the sensor with a number of clock pulses output from the clock pulse generator for the duration of the variation rate signal output from the differentiator, and determining a direction of the skew of the sheet with respect to the transport direction from a result of the comparison. 
     
     
       7. A device as claimed in claim 6, in which the single sensor is located in the vicinity of the rightmost end of the transport path with respect to the transport direction, the computer means being constructed to decide that the sheet is skewed rightwardly upwardly (leftwardly downwardly) when the second predetermined number is larger than the number of pulses which appeared for the duration of the variation rate signal and leftwardly upwardly (rightwardly downwardly) when the second predetermined number is smaller than the number of pulses which appeared for the duration of the variation rate signal. 
     
     
       8. A device as claimed in claim 6, in which the single sensor is located in the vicinity of the leftmost end of the transport path with respect to the transport direction, the computer means being constructed to decide that the sheet is skewed leftwardly upwardly (rightwardly downwardly) when the second predetermined number is larger than the number of pulses which appeared for the duration of the variation rate signal and rightwardly upwardly (leftwardly downwardly) when the second predetermined number is smaller than the number of pulses which appeared for the duration of the variation rate signal.